The is a better way to do this,
Go to the game and create a riot sheild class then go back to the main menu.
Now go to elite edit the riot shield clas to have the riot shield as secondary using overkill then end changes to the game.
Now go back to the game go to your classes go to the riot shield class then go primary and hover ove riot shield wait for changes have been made message, accept then select riot shield,
works with every gun and you can also do alot more with this.